Optical discs which have heretofore been developed include read-only discs such as compact discs (CD), laser discs (LD), etc.; rewritable discs such as magneto-optical discs (MO discs), etc.; and write once discs such as recordable CD (CD-R), etc. FIG. 3 is a perspective view schematically showing one example of such conventional optical discs. As in FIG. 3, in general, an optical disc comprises a substrate with guide grooves (or pits) 1, 2, each having a track pitch P of a predetermined size and a predetermined depth D. For reading or rewriting the information recorded in the optical disc, a laser ray having been focused through a lens to have a wavelength of about 800 nm is irradiated to the pits or to the recorded information on the guide grooves. The information capacity of conventional optical discs is about 640 MB/disc.
Recent developments in the multimedia industry are noticeable. With those, desired are high-density optical discs which can record large-capacity information such as image information and are compact. However, conventional CD and MO discs could not meet the requirements in the market, as their memory capacity is insufficient.
Given that situation, high-density read-only optical discs (so-called DVD) having a diameter of 120 mm, which is the same as that of CD, but having a recording density (4.7 GB) of at least 7 times that of CD are being developed (for example, see "Industrial Materials", Vol. 44, No. 10, pp. 103-105, 1996). DVD comprises a laminate of two substrates, each of which is thinner than the substrate of conventional optical discs. The laser ray to be used for reading the information recorded on DVD has a wavelength of 650 nm or 635 nm. Therefore, the wavelength of the laser ray to be used for reading the information recorded on DVD is shorter than that (about 800 nm) of the laser ray for reading CD. The wavelength of the laser ray to be used for reading the recorded information is in proportion to the spot diameter of the layer ray as focused through a lens. Using a laser ray having a shorter wavelength makes it possible to record and reproduce (or that is, to write and read) higher-density information, as the laser ray shall have a smaller spot diameter. The track pitch P of DVD is 0.74 .mu.m, which is about 1/2 of the track pitch (1.6 .mu.m) of conventional CD. Therefore, the recording density of DVD is greatly increased.
On the other hand, high-density rewritable optical discs (so-called DVD-RAM), of which the guide grooves formed on the substrate have a track pitch P of 1.48 .mu.m and a groove width of 0.74 pm. For information recording on those optical discs, a laser ray having the same wavelength as that for DVD noted above is irradiated to both the inside of the guide grooves (hollows) and the top between the adjacent guide grooves (hills), to thereby accomplish information writing and rewriting through phase conversion (see, for example, Asakura's "DVD", pp. 126-134, published by Ohm Co., 1996). DVD-RAM of that type has the same outer diameter (120 mm) as CD, but has a recording density (2.6 GB on one surface) of about 4 times that of CD. The substrate for DVD-RAM has a thickness of 0.6 mm, and two substrates each having a thickness of 0.6 mm are laminated to construct DVD-RAM. In addition, ultra-high recording-density optical discs comprising a substrate that has a diameter of 120 mm and a thickness of 0.6 mm are being investigated, on which information is recorded and reproduced with a blue laser ray having a shorter wavelength (about 400 nm).
On the other hand, another trial is being made for increasing the density of optical discs of which the thickness is 1.2 mm like that of conventional CD, etc. For example, rewritable optical discs (CD-R) have been proposed, which have grooves having a track pitch width of from 0.3 to 0.6 .mu.m and a depth of from 170 to 250 nm, or pits having a width of from 0.4 to 0.7 .mu.m and a depth of from 280 to 400 nm formed on a substrate having a thickness of 1.2 mm (see, for example. Japanese Patent Application Laid-Open (JP-A) Hei-9-7232).
For producing substrates for high-density optical discs such as DVD and others which are being much developed in those days, a 2P method (photopolymerization method) is being investigated, which comprises forming a UV-curable monomer layer on a transparent substrate such as a plastic substrate or the like, then airtightly applying a stamper having a reverse pattern for the fine structure of pits (or guide grooves) to the UV-curable monomer layer, and exposing the UV-curable monomer layer to UV rays via the substrate to thereby polymerize and cure the monomers in the layer to form pits (or guide grooves) on the substrate (for example, see JP-A Hei-9-106585). According to the 2P method for producing optical disc substrates, it is possible to make the UV-curable monomer having a low viscosity reach the deepest site of the fine structure for pits (or guide grooves) as formed on the stamper. In that method, the UV-curable monomer having been spread to the deepest site of the fine structure for pits (or guide grooves) is cured, and therefore, it is possible to make the substrates have the fine shape of pits (or guide grooves) transferred thereon with high accuracy. However, the 2P method for producing optical disc substrates is problematic in that the mass-producibility therein is lower than that in an injection-molding method for producing substrates, and that the production costs for it are high.
FIG. 4 is a schematic view showing the production of an optical disc substrate through injection molding, in which a synthetic resin being injected via the nozzle tip of a molding machine is charged into a cavity (optical disc substrate-shaped space). In the injection-molding method for producing optical disc substrates, a stamper 4 having a reverse pattern for the fine structure of pits (or guide grooves) formed on its surface is disposed in a mold 3 to give the cavity 7. In producing optical disc substrate in the method, the mold 3 is controlled at a predetermined temperature and clamped, and a synthetic resin 6 having been injected via the nozzle tip of a molding machine is charged into the cavity 7. After the synthetic resin 6 has reached the deepest end of the cavity 7 (this corresponds to the outer edge of the substrate being formed), it is compressed into the fine structure for pits or others of the stamper. Next, this is kept as such for a predetermined period of time to thereby cool and solidify the entire resin in the cavity including the resin to be the center of the substrate, whereby the fine structure of the stamper is transferred onto the solidified resin. Next, the clamped mold 3 is opened, and the optical disc substrate formed Is taken out of the mold 3. In the injection-molding method for producing optical disc substrates, when the resin having been charged into the cavity is contacted with the wall of the mold (the wall forms the cavity), the heat of the resin is transferred to the cavity wall immediately after the contact. With the decrease in the resin temperature, the viscosity of the resin increases. As a result of the transference of the resin heat to the cavity wall, formed is a cooled and solidified layer 5. With the growth of the layer 5, the cavity 7 is filled with the resin. The problem of the reduction in the pattern transferability due to the formation of the cooled and solidified layer in the injection-molding method noted above is seen also in injection-compression molding for producing high-density optical disc substrates.
The injection-molding method has the advantages of good mass-producibility and low production costs. However, the cooled and solidified layer formed in the method brings about the problem of retarding the pattern transferability and increasing the birefringence of the substrate formed, whereby the quality of the substrate is lowered. Where high-density optical disc substrates having finer pits (or guide grooves) than those of conventional ones are produced in the injection-molding method, the problems to be caused by the formation of the cooled and solidified layer will be more serious.
Specifically, with the increase in the density of the pattern for pits (or guide grooves) formed on the surface of the stamper to be used, the resin could hardly enter the depth of the hollows of the fine structure of the pattern thereby often causing transfer failure. For example, even when substrates for DVD-RAM, which are for recording on both the inside of the guide grooves (hollows) and the top between the adjacent guide grooves (hills) through phase conversion, are intended to be formed under the conventional injection-molding conditions, the resin could hardly enter the space to be between the adjacent guide grooves and therefore good hills could not be formed on the substrates. Even if the substrates with such no good fills thereon are used to produce optical discs, good recording and reproduction on the discs produced is impossible and the discs are failed products.
In order solve this problem, a method for producing thin substrates for optical discs through injection-compression molding has been developed, in which the temperature of the mold is set higher than that in conventional methods and the clamping force for the mold is switched during molding operation (see, for example, JP-A Hei-7-176085). The method may be effective for preventing the increase in the viscosity of the resin being charged into the mold and for reducing the birefringence of the substrates produced. However, for the method, the substrate material is limited to only a specific molding resin. In addition, in this method, since the mold temperature is high, the substrates released from the mold after having been cooled and solidified are easily deformed. Moreover, still another problem with the method is that the cooling time in the method must be long and the molding cycle is prolonged.
On the other hand, it is written in "Reports in Optical Memory Symposium '86", page 173 and the following pages, that (1) a polycarbonate resin is injection-molded into optical disc substrates (CD substrates) having a diameter of 130 mm and a thickness of 1.2 mm, at a resin filling rate of 79 cm.sup.3 /sec, and (2) a polymethyl methacrylate resin is injection-molded into optical disc substrates (CD substrates) having the same diameter and thickness as above, at a resin filling rate of 61 cm.sup.3 /sec. In "Polymer Reports", Vol. 49, No. 8, page 703 and the following pages, a reference is made to the production of CD substrates. Briefly, they say therein that the increase in the resin filling rate in producing CD substrates is effective for improving the pattern transferability onto the substrates. However, there is found no literature that suggests the relationship between the pattern transferability and the resin filling rate in injection-molding for producing high-density optical disc substrates, such as DVD substrates which are more small-sized and thinner than conventional optical disc substrates. Further the technologies described in above two literature are related to disc substrates (CD substrates) having a thickness of 1.2 mm. In case of injection molding of such thick substrates at a resin filling rate of about 60 cm.sup.3 /sec, resin filling time is required about 2 seconds, so the resin temperature decreases and the viscosity of the resin increases while the resin is filled up. The increase of the viscosity of the resin causes the lowered pattern transferability.